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Motor Ratings, Speed Control and Positioners

Summary

This chapter covers motor ratings and features beyond the basics: intended duty, premium efficiency, voltage and phase, synchronous speed, temperature rise, thermal protection, and winding RTDs; severe duty, hazardous location, and inverter-duty motors; and variable frequency drives for speed control. It then covers how motors are mounted and adjusted: slide bases, pivot bases, post mount bases, and bi-directional, tri-directional, and jack-screw motor positioners. After completing it, readers can specify a motor's ratings and mounting for a belt-driven fan.

Concepts Covered

This chapter covers the following 18 concepts from the learning graph:

Concept Concept Impact Score
Motor Winding RTD 2
Variable Frequency Drive 69
Motor Positioner 5
Motor Intended Duty 51
Premium Efficiency Motor 25
Motor Voltage and Phase 2
Motor Temperature Rise 2
Bi-Directional Positioner 1
Tri-Directional Positioner 1
Vertical Jack Screws 1
Severe Duty Motor 25
Hazardous Location Motor 25
Synchronous Motor Speed 1
Motor Thermal Protection 1
Inverter-Duty Motor 25
Post Mount Motor Base 1
Slide Base 6
Pivot Motor Base 3

Prerequisites

This chapter builds on concepts from:

The Rest of the Motor Line

Breeze waving welcome A motor line on an order is short, but every field on it decides whether the fan starts, survives its environment and stays aligned. By the end of this chapter you will be able to write that line for a belt-driven fan, choose how the motor is mounted and adjusted, and explain each choice to a customer or a plant electrician. Let's follow the airflow.

Chapter 14 introduced the motor itself: induction and EC technology, horsepower, enclosure, frame size, service factor, insulation class and efficiency class. This chapter finishes the job. FanPedia gives a typical motor specification as a single line (FanPedia p. 71):

50 HP, 1800 RPM, IN, P, 3 Phase, 60 Frequency, 460 Volt, SD, TEFC, 326T

You already know how to read the horsepower, the technology code (IN for induction), the enclosure (TEFC) and the frame (326T). The remaining fields are the subject of the first half of this chapter: the speed (1800 RPM), the efficiency designation (P for premium), the electric power (phase, frequency and voltage), and the intended duty (SD for severe duty). Along the way we add the ratings and protective devices that keep a motor from overheating, and the variable frequency drive that lets a fixed-speed motor run at other speeds. The second half turns to the hardware that holds the motor on a belt-driven fan: slide bases, pivot bases, post mounts and positioners.

The table below lines up each field of the FanPedia example with the section of this book that explains it, so you can see where the gaps are before we fill them.

Field in the example Meaning Where it is covered
50 HP Rated power Chapter 14
1800 RPM Synchronous speed This chapter
IN Induction technology Chapter 14
P Premium efficiency This chapter
3 Phase, 60 Frequency, 460 Volt Electric power This chapter
SD Intended duty: severe duty This chapter
TEFC Enclosure Chapter 14
326T NEMA frame size Chapter 14

Synchronous Motor Speed

An induction motor's stator windings create a magnetic field that rotates around the motor. The speed of that rotating field is the synchronous motor speed, and it depends on only two things: the frequency of the power supply in hertz (Hz, cycles per second) and the number of magnetic poles wound into the stator. Poles always come in pairs, so a motor has 2, 4, 6 or 8 poles, and so on.

\[ n_s = \frac{120 \times f}{p} \]

where:

  • \( n_s \) is the synchronous speed in revolutions per minute (RPM)
  • \( f \) is the supply frequency in Hz
  • \( p \) is the number of poles

A 4-pole motor on 60 Hz power runs its field at \( 120 \times 60 / 4 = 1800 \) RPM, which is the "1800 RPM" in the FanPedia example. The same motor on 50 Hz power would run at 1500 RPM. FanPedia lists the typical synchronous speeds by frequency (FanPedia p. 71):

Poles 60 Hz 50 Hz
2 3600 RPM 3000 RPM
4 1800 RPM 1500 RPM
6 1200 RPM 1000 RPM
8 900 RPM 750 RPM

The rotor of an induction motor always turns a little slower than its field. That small difference, called slip, is what induces current in the rotor and produces torque. This is why a motor ordered as "1800 RPM" shows a somewhat lower full-load speed on its nameplate, and FanPedia notes that actual nominal motor speeds vary by motor horsepower and manufacturer. On an order, the motor speed is stated as the synchronous speed; the fan selection uses the actual operating speed.

Motor Voltage and Phase

The electric power line of a motor specification has three parts. Phase is single phase (one alternating voltage, common in small commercial equipment) or three phase (three alternating voltages offset in time, the standard for industrial motors). Frequency is 60 Hz in North America and 50 Hz in much of the rest of the world. Voltage is the nominal supply voltage at the motor terminals. The fan industry generally uses AC power for induction motors, and DC power for EC motors and some light commercial accessories and damper motors (FanPedia p. 72).

FanPedia lists the common combinations (FanPedia p. 72):

Power Phase Frequency Voltages
AC Single 60 Hz 115, 230 or 277
AC Single 50 Hz 220 or 240
AC Three 60 Hz 200, 208, 230, 400, 440, 460, 575, 2300, 4160 or 4300
AC Three 50 Hz 190, 380, 415 or 4000
DC – – 12, 24, 28, 36, 48, 72, 90, 150, 180, 240, 300 or 500

Many motors are dual voltage, such as 115/230, 208/230 or 230/460, and can be wired for either. That flexibility creates a trap on orders with electrical add-ons, which the next note explains.

A Dual-Voltage Motor Still Needs One Voltage

Breeze warning It is tempting to write "230/460 V" on an order and let the field decide, but a disconnect switch or speed controller supplied with the fan is built for one voltage. FanPedia says the customer needs to state the actual voltage the fan will use whenever the motor has add-ons (FanPedia p. 72). Ask for the site voltage before you release the order, not after the equipment arrives.

Premium Efficiency Motor

Motor efficiency is the ratio of the useful shaft power a motor delivers to the electrical power it draws; the difference leaves the motor as heat. Chapter 14 introduced efficiency classes in general. Twin City Fan uses two designations based on the NEMA classifications (FanPedia p. 72):

  • Standard Efficiency, which compares to NEMA High Efficiency (and to IEC IE2)
  • Premium Efficiency, which compares to NEMA Premium Efficiency (and to IEC IE3)

A premium efficiency motor is therefore the "P" in the example line: a motor built with more and better active material (copper, electrical steel) so that less of its input becomes heat. FanPedia also notes that IE1 is the IEC standard-efficiency level and that IE4 (super premium) is a future designation not currently used.

Efficiency also depends on how heavily the motor is loaded. FanPedia states that most motors are designed to run at 50% to 100% of rated load, that maximum efficiency is usually near 75% of rated load, and that efficiency drops sharply below about 50% load. A 10 HP motor therefore has an acceptable load range of 5 to 10 HP, with peak efficiency near 7.5 HP (FanPedia p. 72). An oversized premium motor running at 30% load can waste more energy than a correctly sized standard motor.

Worked example: what does "P" buy? A belt-driven fan needs 20 brake horsepower (bhp) at its duty point and runs 6,000 hours a year. The selection uses a 25 HP motor, so the motor runs at \( 20 / 25 = 80\% \) load, inside the 50% to 100% band and close to the peak-efficiency point. For this illustration, assume the standard efficiency motor is 92.4% efficient at this load and the premium motor is 93.6% (illustrative values; real values come from the motor data sheet).

  1. Convert the shaft power to kilowatts: \( 20 \times 0.7457 = 14.91 \) kW.
  2. Electrical input, standard motor: \( 14.91 / 0.924 = 16.14 \) kW.
  3. Electrical input, premium motor: \( 14.91 / 0.936 = 15.93 \) kW.
  4. Difference: \( 16.141 - 15.934 \approx 0.207 \) kW (steps 1 to 3 are rounded for display; carry three decimals here, from 14.914 kW).
  5. Annual energy saved: \( 0.207 \times 6000 \approx 1{,}240 \) kWh per year.

At an illustrative $0.10 per kWh, that is about $124 a year for one fan. The number is small per fan but recurs every year of the motor's life, and it scales with horsepower and run hours. That is why premium efficiency is the usual request on continuously running fans and why many specifications require it outright.

Heat: Temperature Rise, Thermal Protection and Winding RTDs

Every watt a motor loses becomes heat in its windings, and heat is what ages motor insulation. Three related ideas describe and control that heat.

Motor Temperature Rise

Motor temperature rise is the change in temperature from the ambient air around the motor to the motor's steady-state operating temperature (FanPedia p. 69). The rise is a property of the motor design and its load; the ambient is a property of the site. Add them and you get the winding temperature, which must stay within the limit of the insulation class from Chapter 14. FanPedia notes that the typical fan motor is built with Class F insulation but rated for a Class B temperature rise. The motor therefore runs cooler than its insulation could tolerate, and that margin is what lets it survive a hot day, a dirty cooling fan, or a period on the service factor. When a motor sits in a hot location, the ambient temperature rating (the maximum allowable temperature around the motor) has to be checked along with the rise.

Motor Thermal Protection

Motor thermal protection is a device built into the motor that responds to winding temperature. FanPedia lists thermistors and thermostats as the common choices and says they protect the motor against locked-rotor conditions, continuous overload and high ambient temperature (FanPedia p. 75). A thermostat is a switch that opens at a set temperature; a thermistor is a resistor whose resistance changes sharply at a set temperature and is read by a relay. Either one gives the control circuit a signal to shut the motor down before the insulation is damaged. Do not confuse thermal protection with a space heater (also called a strip heater), which warms an idle motor to prevent condensation.

Motor Winding RTD

A motor winding RTD is a resistance temperature detector embedded in the motor windings to sense their heat rise (FanPedia p. 75). Where a thermostat or thermistor answers a yes-or-no question (too hot or not), an RTD reports a continuous temperature, because its electrical resistance changes in a known way with temperature. Plants with monitoring systems use winding RTDs to trend motor temperature, alarm at one value and trip at a higher one. RTDs are also available for motor bearings, and Chapter 16 covers bearing RTDs on the fan itself. Winding RTDs are a motor modification, so they must be called out on the order; they are more common on large induction motors in process plants.

Match the Device to the Plant's Control System

Breeze giving a tip Before choosing between thermostats, thermistors and winding RTDs, ask one question: "What will this signal be wired to?" A plant with a monitoring system wants RTDs it can trend; a simple starter circuit only needs a switch that opens.

Motor Intended Duty

Motor intended duty describes the operating conditions a motor is built for, based on its surrounding environment (FanPedia p. 71). It is the "SD" in the example line, and it is separate from the enclosure: the enclosure says how the motor is sealed and cooled, while the intended duty says how well the whole motor (paint, seals, hardware, bearings, conduit box) stands up to its surroundings. FanPedia lists these duty designations (FanPedia p. 73):

  • GD (General Duty): general purpose motors used where the air around the motor is normal, clean and cool.
  • SD (Severe Duty): used where a severe duty motor is needed but IEEE 841 motors are not specified. Also called Mil/Chem duty or chemical duty.
  • IEEE 841: an industry standard for severe duty motors begun in 1986, used in process industries.
  • DFG: explosion proof enclosure motors used in hazardous locations.
  • Special basis: IEEE 45 motors for marine applications and washdown duty motors for food applications. IEEE 45 is not a motor-manufacturing standard: it is the IEEE series Recommended Practice for Electrical Installations on Shipboard, and Twin City Fan describes the resulting option as "IEEE 45 marine-duty motors for both above deck and below deck installations" (tcf.com marine page). Motor makers sell "IEEE 45 marine duty" motors as TEFC, usually IP56, with severe-duty construction. Because the ship's design practice itself recommends IEEE 841 motors for 10 to 500 hp (IEEE Std 45.1-2017, 13.2), a marine motor often carries IEEE 841 features too. Confirm the exact motor build and enclosure with Twin City Fan when a marine specification is involved.

Enclosure and duty have to agree. The motor specifications page pairs them (FanPedia p. 73); the NEMA side of that table is summarized below.

Enclosure Intended duties offered
ODP GD
TEFC GD or SD; IEEE 841 and IEEE 45 (marine, special basis) in the IP56 version
EXPL (explosion proof) DFG
TENV GD or SD

Worked example: choosing the duty. Three inquiries arrive in one week.

  1. A school's air handler fan; the motor sits in a clean, cool mechanical room. Normal, clean, cool air is the definition of general duty, so the motor is GD, and an ODP or TEFC enclosure both fit.
  2. A chemical plant asks for a "chem duty" motor on a scrubber exhaust fan; its specification does not mention IEEE 841. Chemical duty is one of the names FanPedia gives for SD, so the motor is TEFC, SD.
  3. A refinery specification calls for "IEEE 841 motors on all rotating equipment." The duty is IEEE 841, which forces a TEFC (IP56) enclosure. Quoting a plain SD motor here would not meet the specification, even though both are severe duty.

The next two sections look more closely at the two duties in this list that carry the most weight on industrial orders.

Severe Duty Motor

A severe duty motor is built to keep running where a general duty motor would corrode, wear or overheat early: around moisture, chemical fumes, dust, washdown or outdoor exposure. FanPedia uses "SD" for the general severe duty motor and lists Mil/Chem duty and chemical duty as names you will see for it (FanPedia p. 73). IEEE 45 is not another name for SD: the same page files it under the special-basis duties for marine applications, and pairs it with SD only in the TEFC IP56 row of its enclosure table. IEEE 841 is a stricter, published standard for severe duty motors in process industries; FanPedia names automotive, mining, paper and wood mills, and refining as typical users.

The rule for choosing between them comes straight from FanPedia's definition: SD is used where a severe duty motor is required but IEEE 841 is not specified. That makes the decision a reading exercise, not an engineering judgment.

Worked example: SD or IEEE 841? An automotive paint shop needs an exhaust fan. The motor will sit near the fan in air that carries paint mist and dirt, so general duty is out. Read the customer's specification:

  1. If it says "IEEE 841" anywhere in the motor section, quote an IEEE 841 motor (TEFC, IP56).
  2. If it says "severe duty," "mill and chemical duty" or describes the environment without naming IEEE 841, quote an SD motor (TEFC).
  3. If the specification is silent and the motor is in the dirty air, recommend SD and say why on the quotation.

In this case the specification reads "TEFC, severe duty, premium efficiency," so the motor line is: TEFC, SD, P. Had the same plant's corporate standard required IEEE 841, the answer would change even though the air did not.

Hazardous Location Motor

A hazardous location is an area where flammable gases or vapors, or combustible dusts, may be present in amounts that can ignite. The U.S. National Electrical Code (NEC) classifies these areas by class (Class I for gases and vapors, Class II for dusts), by division (whether the hazard is present in normal operation or only under abnormal conditions) and by group (the specific family of gas or dust). The plant owner, not the fan supplier, determines the classification of each area.

A hazardous location motor is a motor built and labeled for one of those classifications. In FanPedia's terms it is a DFG motor with an explosion proof (EXPL) enclosure: the internal portion of the motor is engineered to keep a motor arc or explosion from igniting a surrounding atmosphere that contains flammable dust or gas, while an external fan cools it (FanPedia p. 73). FanPedia names Groups D (gases), F and G (dusts) as the groups commonly used at Twin City Fan, and directs readers to the NEC for the full list of gases and dusts and their class and division ratings. Applications under the European ATEX directive require a special motor.

Worked example: reading the classification onto the order. A food plant requests a fan for a room the owner's drawings mark as Class II, Division 1, Group G (combustible food dust).

  1. The area is hazardous, so the motor must be a hazardous location motor: enclosure EXPL, duty DFG.
  2. The motor nameplate must carry the same class, division and group the owner gave: Class II, Division 1, Group G. Copy those values from the owner's documents onto the order; do not choose them yourself.
  3. The motor is only part of the answer. The fan also needs a review for spark-resistant construction, covered in Chapter 20.

An Explosion Proof Motor Does Not Make a Spark-Proof Fan

Breeze warning A common mistake is to order a DFG motor and assume the fan is now safe for the hazardous area. The motor rating covers the motor's own arcs and heat; a rubbing impeller or a dragging belt can still strike a spark. Treat the motor classification and the fan's spark-resistant construction as two separate lines that both need an answer.

The sorter below gives you practice with all of the duty designations from the last three sections. Each case states only what an application engineer would learn from the inquiry.

Diagram: Motor Intended Duty Sorter

Run Motor Intended Duty Sorter Fullscreen

Motor Intended Duty Sorter

Type: microsim sim-id: ch15-motor-intended-duty-sorter
Library: p5.js
Status: Specified
Bloom Level: Evaluate
Bloom Verb: judge
Learning Objective: The learner will judge which motor intended duty (GD, SD, IEEE 841, DFG or Washdown) an application requires, given a short description of the motor's environment and the customer's specification, with at least 7 of 8 cases correct on the first attempt.

Prerequisites: motor intended duty, general duty, severe duty motor, IEEE 841, hazardous location motor, DFG, washdown duty (all defined in the sections "Motor Intended Duty", "Severe Duty Motor" and "Hazardous Location Motor" above).

Evidence of Mastery: The learner assigns each of the 8 cases to one of five duty bins and commits each assignment with a Check action. An assignment is correct when it matches the "Correct duty" column in Content. Mastery is 7 of 8 correct on the first attempt. Reading the bin definitions is exploration, not evidence.

Misconceptions: (1) Any severe environment calls for IEEE 841, when IEEE 841 is used only when it is specified. (2) A dusty or dirty environment is automatically a hazardous location, when only the owner's area classification makes it one. (3) General duty is acceptable anywhere the motor has a TEFC enclosure.

Instructional Rationale: Choosing a duty is a judgment against stated criteria, so a classification sorter with one committed answer per case and criterion-based feedback makes the learner apply FanPedia's rules rather than recall a list.

Content:

The five bins, each with the definition shown when the learner opens it:

Bin Definition shown
GD (General Duty) General purpose motor for normal, clean and cool air around the motor.
SD (Severe Duty) Used where a severe duty motor is needed but IEEE 841 is not specified; also called Mil/Chem duty or chemical duty.
IEEE 841 Industry standard for severe duty motors begun in 1986, used in process industries when specified.
DFG Explosion proof enclosure motor for a hazardous location classified under the NEC.
Washdown Special-basis motor for food applications that are washed down.

The eight cases, in this order:

# Case shown to the learner Correct duty Why (shown as feedback)
1 Office building supply fan. The motor is in a clean, cool mechanical room. GD Normal, clean and cool air around the motor is the definition of general duty.
2 Paper mill process fan. The mill's motor standard requires IEEE 841 motors. IEEE 841 IEEE 841 is chosen when the customer specifies it, and paper mills are a typical user.
3 Chemical plant scrubber exhaust fan. The specification asks for a "chemical duty" motor and does not mention IEEE 841. SD Chemical duty is another name for severe duty, and IEEE 841 was not specified.
4 Exhaust fan for a room the owner classifies as Class I, Division 1, Group D (flammable gas). DFG A classified hazardous location needs an explosion proof DFG motor rated for that class and group.
5 Food plant fan that is hosed down with the rest of the line during cleaning. Washdown FanPedia lists washdown duty motors, ordered on a special basis, for food applications.
6 Grain handling room the owner classifies as Class II, Division 1, Group G (combustible dust). DFG Group G dust in a classified area needs an explosion proof DFG motor.
7 Automotive plant exhaust fan. The motor sits in air with oil mist and dirt; the specification says "severe duty" and does not mention IEEE 841. SD The environment is severe, but IEEE 841 was not specified, so FanPedia's rule gives SD.
8 Mining process fan. The specification says "IEEE 841 TEFC motors." IEEE 841 IEEE 841 is specified, and mining is one of the process industries that use it.

Provenance: Bin definitions and the rules behind each answer: FanPedia p. 73 (Motor Specifications, Intended Duty and Enclosure), as presented in this chapter's sections "Motor Intended Duty", "Severe Duty Motor" and "Hazardous Location Motor". The NEC class, division and group labels in cases 4 and 6 are standard NEC terms; the cases themselves are illustrative and must be labeled "Illustrative cases".

Rules: Each case has exactly one correct bin. A case that is both severe and hazardous does not occur in this set. A case that names IEEE 841 is always IEEE 841; a severe case that does not name IEEE 841 is always SD.

Learner Activity:

  1. The learner reads case 1 and opens any bin definitions they want.
  2. The learner places the case in one bin and presses Check. The case is marked correct or incorrect and the feedback text appears.
  3. The learner moves to the next case. Cases already checked stay marked and cannot be changed.
  4. After case 8, a summary lists all eight cases with the learner's bin, the correct bin and the reason.

Feedback: Eight cases, fixed order, one attempt each. Correct: "Correct: . ". Incorrect: "Not quite. The correct duty is . ". The correct answer is revealed immediately after each Check. A running score "Correct: n of 8" is shown, and the summary states whether the 7-of-8 mastery level was reached.

Starting State: Case 1 is shown with all five bins closed and no score. The prompt reads "Which intended duty does this motor need?"

Chapter Anchors: The chapter names five duty bins: GD, SD, IEEE 841, DFG and Washdown. IEEE 841 began in 1986. SD is used where IEEE 841 is not specified. FanPedia lists Groups D, F and G as common at Twin City Fan.

Variable Frequency Drive

A variable frequency drive (VFD) is an electronic controller placed between the power supply and an induction motor. It converts fixed-frequency AC power into AC power at a frequency it can adjust, typically from a small fraction of line frequency up to line frequency. Because synchronous speed is set by frequency, changing the frequency changes the motor speed in direct proportion, and with it the fan speed. On a belt-driven fan the sheaves still set the ratio between motor and fan speed; the VFD changes both together.

FanPedia treats the VFD as one of the two usual ways to change fan performance in the field, alongside variable inlet vanes (FanPedia p. 170). Chapter 18 covers the mechanical flow-control devices; this section covers what the VFD does to the fan.

Frequency Is the Speed Knob

Breeze thinking Notice that nothing about the motor's poles or the fan's sheaves changes on a VFD. The drive moves the one variable in \( n_s = 120 f / p \) that the plant normally treats as fixed, and the fan laws do the rest.

The fan laws (developed fully in Chapter 25) say how a fan's performance changes with speed on a fixed system. With \( N \) the fan speed and the subscripts 1 and 2 for the old and new conditions:

\[ \frac{Q_2}{Q_1} = \frac{N_2}{N_1} \qquad \frac{P_2}{P_1} = \left(\frac{N_2}{N_1}\right)^2 \qquad \frac{H_2}{H_1} = \left(\frac{N_2}{N_1}\right)^3 \]

where \( Q \) is airflow (CFM), \( P \) is pressure (in. wg) and \( H \) is brake horsepower. On a VFD, the speed ratio equals the frequency ratio, \( N_2 / N_1 = f_2 / f_1 \).

Worked example: turning a fan down to 45 Hz. A belt-driven fan is selected for 10,000 CFM at 2.00 in. wg static pressure, needing 8.0 bhp at full speed on 60 Hz power (illustrative values). The owner wants to run it at reduced airflow overnight, and the VFD is set to 45 Hz.

  1. Speed ratio: \( 45 / 60 = 0.75 \). A 4-pole motor's synchronous speed drops from 1800 to 1350 RPM, and the fan speed drops by the same 25%.
  2. Airflow: \( 10{,}000 \times 0.75 = 7{,}500 \) CFM.
  3. Static pressure: \( 2.00 \times 0.75^2 = 2.00 \times 0.5625 = 1.13 \) in. wg.
  4. Brake horsepower: \( 8.0 \times 0.75^3 = 8.0 \times 0.4219 = 3.38 \) bhp.

A 25% cut in airflow cuts the fan's power by more than half. That cube relationship is the reason VFDs are so common on fans that spend long hours below full flow. Two limits keep the example honest. The fan laws assume the system resistance follows the fan along a system curve that passes through zero, so a system with a fixed pressure requirement will not fall off this steeply. And a VFD can drive a motor above 60 Hz, but the fan must never be run above the maximum safe speed listed in the current catalog; any speed increase starts with that check.

The calculator below lets you test these relationships at other frequencies before you see the answers.

Diagram: VFD Frequency and Fan Law Calculator

Run VFD Frequency and Fan Law Calculator Fullscreen

VFD Frequency and Fan Law Calculator

Type: microsim sim-id: ch15-vfd-fan-law-calculator
Library: p5.js
Status: Specified
Bloom Level: Apply
Bloom Verb: calculate
Learning Objective: The learner will calculate the airflow, static pressure and brake horsepower of a belt-driven fan at a reduced VFD frequency, using the fan laws with the frequency ratio as the speed ratio, to within 2% of the model values.

Prerequisites: variable frequency drive, synchronous motor speed, poles, frequency, airflow (CFM), static pressure (in. wg), brake horsepower, fan laws (defined in the sections "Synchronous Motor Speed" and "Variable Frequency Drive" above and in Chapter 25).

Evidence of Mastery: For each of three challenge frequencies, the learner types predicted airflow, static pressure and brake horsepower before the sim reveals the model values. A prediction is correct when it is within ±2% of the model value. Mastery is all nine values correct (three per challenge) within two attempts. Moving the frequency control in free exploration is not evidence.

Misconceptions: (1) Power falls in the same proportion as frequency (a 25% cut in frequency giving a 25% cut in power). (2) Pressure is unchanged by a speed change. (3) The VFD changes the motor's number of poles or the sheave ratio.

Instructional Rationale: Apply-level calculation needs practice with a check, so each challenge requires a committed prediction before the answer appears; free exploration afterward lets the learner see the square and cube curves across the whole frequency range.

Content:

Base duty, fixed and labeled "Illustrative values": 10,000 CFM, 2.00 in. wg static pressure, 8.0 bhp at 60 Hz. Motor: 4 poles.

The adjustable quantity:

Quantity Min Max Step Default Unit
Drive frequency f 20 60 1 60 Hz

Displayed at every frequency: frequency ratio f/60, synchronous speed 120 × f / 4 in RPM, airflow, static pressure, brake horsepower, and each as a percent of its 60 Hz value.

Challenge frequencies, in this order:

# Frequency (Hz) Airflow (CFM) Static pressure (in. wg) Brake horsepower Feedback when wrong
1 45 7,500 1.13 3.38 Ratio 45/60 = 0.75. Airflow × 0.75; pressure × 0.75² = 0.5625; power × 0.75³ = 0.4219.
2 30 5,000 0.50 1.00 Ratio 0.5. Pressure × 0.25 and power × 0.125: half the speed needs one eighth of the power.
3 50 8,333 1.39 4.63 Ratio 50/60 = 0.8333. Pressure × 0.6944; power × 0.5787.

Provenance: Base duty values are illustrative (this chapter, "Variable Frequency Drive" worked example). Synchronous speed formula and the 1800 RPM, 4-pole, 60 Hz value: FanPedia p. 71 and this chapter's "Synchronous Motor Speed" section. Fan laws: standard fan engineering, as stated in this chapter and Chapter 25.

Rules: r = f / 60. Synchronous speed = 120 × f / 4 RPM. Airflow = 10,000 × r CFM. Static pressure = 2.00 × r² in. wg. Brake horsepower = 8.0 × r³ bhp. Display airflow rounded to the nearest 1 CFM, pressure to 0.01 in. wg, power to 0.01 bhp. A prediction is correct when |prediction − model| <= 0.02 × model. The model ignores motor slip, drive losses and any fixed system pressure; the sim states this in one line beneath the results. Frequencies above 60 Hz are not offered, and the sim states that operation above 60 Hz requires checking the fan's maximum safe speed in the current catalog.

Learner Activity:

  1. The learner reads challenge 1 (45 Hz) and types predicted airflow, static pressure and brake horsepower.
  2. The learner presses Check. Each of the three values is marked correct or incorrect against the model.
  3. After all three challenges, free exploration unlocks: the learner changes the frequency from 20 to 60 Hz and every displayed value updates at once, along with a plot of percent airflow, percent pressure and percent power against frequency.
  4. The learner should notice that at 30 Hz airflow is still 50% while power is 12.5%.

Feedback: Three challenges, fixed order, two attempts each; on each attempt all three values are checked together. Correct: "Correct: at Hz." Hint after a first wrong attempt: each value is marked ✓ or ✗, and each wrong value gets one line that names the slip without a ratio, multiplier or model value: still the 60 Hz value ("scale it to the new frequency"), divided by the ratio instead of multiplying, or the wrong power of the ratio ("you used the ratio itself"), each followed by the rule (airflow goes with r, pressure with r², power with r³); otherwise "Recheck the arithmetic: r = f / 60, then …". After the final attempt: the model values, the "Feedback when wrong" text for that challenge and the worked calculation are shown, and the challenge counts as missed. A running count "Challenges correct: n of 3" is shown.

Starting State: Challenge 1 is shown with empty answer boxes, the frequency set to 60 Hz and the base duty displayed. The prompt reads "The drive is turned down to 45 Hz. What will the fan deliver, and what will it draw?"

Chapter Anchors: Base duty 10,000 CFM, 2.00 in. wg, 8.0 bhp at 60 Hz. At 45 Hz: speed ratio 0.75, synchronous speed 1350 RPM, 7,500 CFM, 1.13 in. wg, 3.38 bhp. A 4-pole motor runs at 1800 RPM synchronous on 60 Hz.

Inverter-Duty Motor

A VFD does not produce a smooth sine wave. It builds its output from rapid switched voltage pulses, and those pulses can stress motor insulation with voltage spikes and can induce currents in the motor shaft that discharge through the bearings. An inverter-duty motor is an induction motor designed and rated for operation on a VFD (the drive's output stage is called an inverter), with insulation and construction meant to withstand that power. The governing reference is NEMA MG-1 Part 31, which FanPedia cites when it notes that precautions may be needed to eliminate or reduce shaft currents imposed on a motor by a VFD (FanPedia p. 72).

FanPedia lists the motor modifications that address shaft currents: insulated bearings, whose outer race is coated with insulating material, and a shaft grounding ring, internal or external, that gives the induced charge a path to ground around the bearings (FanPedia p. 75; see also Chapter 14). FanPedia also notes that a common service factor for a motor operated on a VFD is 1.0 (FanPedia p. 69), so the service factor margin should not be counted on when a fan runs on a drive.

Worked example: will a TEFC motor stay cool at low speed? A TEFC motor is cooled by an external fan on its own shaft, so when the VFD slows the motor, the cooling air slows too. Take the fan from the previous example and turn the drive down to 30 Hz.

  1. Speed ratio: \( 30 / 60 = 0.5 \). The motor's cooling fan moves roughly half its normal air.
  2. Fan load: \( 0.5^3 = 0.125 \), so the motor delivers about 12.5% of its full-speed power.
  3. Because fan load falls with the cube of speed while cooling falls roughly in proportion to speed, a centrifugal fan's motor generally runs with less heat to remove than it has cooling for, even at low speed.

The conclusion is not that any motor will do. It is that, for a fan, the main reasons to order an inverter-duty motor are the drive's voltage spikes and shaft currents, not low-speed heating; the heating case is more serious for loads that keep their torque at low speed. On the order, state that the motor will run on a VFD, request an inverter-duty rating, and decide with the customer whether insulated bearings or a shaft grounding ring are needed.

Mounting the Motor on a Belt-Driven Fan

A motor base is the structure that mounts the motor to the fan or to the fan base on a belt-driven fan. FanPedia describes it as the means for positioning the motor and adjusting belt tension during installation and maintenance, and lists the styles used depending on fan design and industry requirements: slide base or slide rails, post mount, bolt-on and various pivot styles (FanPedia p. 189). Belt tension is set by changing the center distance between the motor sheave and the fan sheave, so every style below is, at heart, a way to move the motor a controlled distance and then lock it in place.

Slide Base

A slide base is a motor base in which the motor bolts to a plate or frame that slides along a fixed base, with an adjusting screw that pushes the motor away from or toward the fan to set belt tension. FanPedia distinguishes two forms (FanPedia p. 76):

  • NEMA type slide base, used on small to large motors from 48 to 445 frame. It mounts on the fan base or on the fan pedestal. Typical mounting: Arrangements 1 and 3 (floor mounted or on a fan base); Arrangements 9, 9F, 9H and 9ST; plenum fans in Arrangements 1, 3, 3HS, 3VS and 3SM; and pedestal plug fans in Arrangements 1P and 9P.
  • Heavy-duty slide rails, two rails per motor, used on large to very large motors in the 440, 500 and 5000 frame series. They mount on a base, or on the pedestal for Arrangement 9H only.

On some Arrangement 9 fans, Unistrut channels on the pedestal give the slide base a wider range of belt center adjustment (FanPedia pp. 76 and 85).

Worked example: the motor from the FanPedia line. The example motor is a 50 HP, 326T frame. Suppose it drives an Arrangement 9 housed centrifugal fan.

  1. Check the frame against the NEMA slide base range: 326 lies between 48 and 445, so the NEMA type slide base fits.
  2. Check the arrangement: Arrangement 9 is on the slide base's typical mounting list.
  3. Result: NEMA type slide base on the pedestal.

Now change the motor to a 5008S frame (one of FanPedia's large-horsepower special sizes, FanPedia p. 74) on an Arrangement 1 fan. The 5000 series is outside the NEMA slide base range, so the answer becomes heavy-duty slide rails on the base. A 449T motor lands in the same place: it is a 440-series frame but above the 445 limit of the NEMA slide base.

Pivot Motor Base

A pivot motor base carries the motor on a plate hinged at one edge, so the motor swings about the pivot point and belt tension is set by the swing rather than a straight slide. FanPedia's automotive pivot bases are used mostly in automotive applications, on Arrangement 9H pedestals, Arrangement 9SS swingout fans and, specially mounted on a fan base, Arrangement 3 (FanPedia p. 77). Because the pivot must sit on the correct side of the motor, motor location is restricted by rotation: for Arrangements 9H and 9SS, clockwise rotation goes with motor position L and counterclockwise with R; for Arrangement 3, clockwise goes with motor position Z and counterclockwise with W. Motor positions and rotation are covered in Chapter 12. A separate pivot/bolted adjustable base is used on larger motors, 254 to 445 frame, on tubular centrifugal and axial Arrangement 9 fans, plenum Arrangements 3HA and 3VA, and plug fan Arrangement 9 (FanPedia p. 78).

Post Mount Motor Base

A post mount motor base is an adjustable base in which the motor plate is carried on posts, typically threaded, so the motor can be moved along the posts to tension the belts. FanPedia's standard post mount is used on small motors, 48 to 215 frame, in a saddle mount or flat mount version, on the same fan types as the pivot/bolted design: tubular centrifugal and axial Arrangement 9, plenum Arrangements 3HA and 3VA, and plug fan Arrangement 9 (FanPedia p. 78). The two are a matched pair split by frame size: post mount up to 215 frame, pivot/bolted from 254 to 445. For tubeaxial, vaneaxial and centaxial Arrangement 9 fans, FanPedia also shows a bolt-on mount for 48 to 286 frame motors in a saddle mount.

The table below gathers the base styles from these three subsections by frame range.

Base style Motor frame range Typical mounting (FanPedia)
NEMA type slide base 48 to 445 Arr. 1, 3, 9, 9F, 9H, 9ST; plenum 1, 3, 3HS, 3VS, 3SM; plug 1P, 9P
Heavy-duty slide rails 440, 500 and 5000 series On base; on pedestal for Arr. 9H only
Automotive pivot base Not stated by frame Arr. 9H, 9SS, specially mounted Arr. 3; mostly automotive
Standard post mount 48 to 215 Tubular centrifugal and axial Arr. 9; plenum 3HA, 3VA; plug Arr. 9
Pivot/bolted adjustable base 254 to 445 Same as standard post mount
Bolt-on mount 48 to 286 Tubeaxial, vaneaxial, centaxial Arr. 9

Diagram: Motor Base Selector

Run Motor Base Selector Fullscreen

Motor Base Selector

Type: microsim sim-id: ch15-motor-base-selector
Library: p5.js
Status: Specified
Bloom Level: Apply
Bloom Verb: apply
Learning Objective: The learner will apply FanPedia's motor base frame ranges and typical mountings to choose the motor base style for a belt-driven fan, given the motor frame, the fan type and arrangement, and the industry, with at least 7 of 8 cases correct.

Prerequisites: motor base, slide base, heavy-duty slide rails, pivot motor base, automotive pivot base, post mount motor base, pivot/bolted adjustable base, NEMA frame size, fan arrangement (defined in "Mounting the Motor on a Belt-Driven Fan" above and in Chapters 11 and 14).

Evidence of Mastery: For each of 8 cases the learner selects one of five base styles and commits it with Check. A selection is correct when it matches the "Correct base" column in Content. Mastery is 7 of 8 correct on the first attempt. Opening the reference table is exploration, not evidence.

Misconceptions: (1) Any motor up to 449T fits a NEMA slide base, when the NEMA slide base stops at 445 frame. (2) Post mount and pivot/bolted bases are interchangeable, when they are split by frame size (48 to 215 versus 254 to 445). (3) Automotive pivot bases can use any motor position, when rotation restricts motor location.

Instructional Rationale: Choosing a base is a procedure: check frame range, then arrangement, then industry. Eight committed cases with rule-based feedback make the learner run that procedure on varied inputs, which is Apply-level practice.

Content:

The five base styles the learner can choose: NEMA type slide base; Heavy-duty slide rails; Automotive pivot base; Standard post mount; Pivot/bolted adjustable base.

A reference table the learner can open at any time, with these rows:

Base style Frame range Typical mounting
NEMA type slide base 48 to 445 Arr. 1, 3, 9, 9F, 9H, 9ST; plenum 1, 3, 3HS, 3VS, 3SM; pedestal plug 1P, 9P
Heavy-duty slide rails 440, 500 and 5000 series On base; on pedestal for Arr. 9H only
Automotive pivot base Not stated Arr. 9H pedestal, Arr. 9SS swingout, Arr. 3 specially mounted; mostly automotive
Standard post mount 48 to 215 Tubular centrifugal and axial Arr. 9; plenum 3HA, 3VA; plug Arr. 9
Pivot/bolted adjustable base 254 to 445 Tubular centrifugal and axial Arr. 9; plenum 3HA, 3VA; plug Arr. 9

The eight cases, in this order:

# Case shown to the learner Correct base Why (shown as feedback)
1 326T motor on an Arrangement 9 housed centrifugal fan, general industry. NEMA type slide base 326 is within 48 to 445, and Arrangement 9 is on the slide base mounting list.
2 5008S motor on a floor-mounted Arrangement 1 fan. Heavy-duty slide rails The 5000 series is beyond the NEMA slide base range and is listed for heavy-duty slide rails.
3 184T motor on an Arrangement 9 tubular centrifugal fan. Standard post mount 184 is within 48 to 215, the standard post mount range for tubular centrifugal Arrangement 9.
4 286T motor on an Arrangement 3HA plenum fan. Pivot/bolted adjustable base 286 is above 215 and within 254 to 445, the pivot/bolted range for plenum Arrangement 3HA.
5 213T motor on an Arrangement 3VA plenum fan. Standard post mount 213 is within 48 to 215, and plenum Arrangement 3VA is on the post mount list.
6 Arrangement 9SS swingout fan for an automotive plant, clockwise rotation. Automotive pivot base Arrangement 9SS is listed only for automotive pivot bases; with clockwise rotation the motor goes in position L.
7 449T motor on an Arrangement 3 plenum fan. Heavy-duty slide rails 449 is a 440-series frame above the 445 limit of the NEMA slide base.
8 254T motor on an Arrangement 9P pedestal plug fan. NEMA type slide base Pedestal plug fans in Arrangement 9P are on the NEMA slide base mounting list, and 254 is within 48 to 445.

Provenance: Frame ranges and typical mountings: FanPedia p. 76 (Slide Bases/Rails), p. 77 (Automotive Pivot), p. 78 (Adjustable Bases); the 5008S special size: FanPedia p. 74. Cases are illustrative combinations and must be labeled "Illustrative cases".

Rules: Each case has one correct base. Frame number comparisons use the numeric part of the frame designation (326T is 326). NEMA type slide base: 48 <= frame <= 445. Standard post mount: 48 <= frame <= 215. Pivot/bolted adjustable base: 254 <= frame <= 445. Heavy-duty slide rails: 440, 500 or 5000 series; a 440-series frame above 445 has only this choice. Automotive pivot base: Arrangement 9SS, or an automotive application that specifies it.

Learner Activity:

  1. The learner reads case 1 and may open the reference table.
  2. The learner selects a base style and presses Check. The case is marked correct or incorrect and the feedback appears.
  3. The learner moves to the next case; checked cases stay marked and cannot be changed.
  4. After case 8, a summary shows each case, the learner's choice, the correct base and the reason.

Feedback: Eight cases, fixed order, one attempt each. Correct: "Correct: . ". Incorrect: "Not quite. The correct base is . ". The correct answer is revealed immediately after Check. A running score "Correct: n of 8" is shown, and the summary states whether the 7-of-8 mastery level was reached.

Starting State: Case 1 is shown with no base selected and the reference table closed. The prompt reads "Which motor base fits this fan and motor?"

Chapter Anchors: NEMA type slide base 48 to 445 frame. Heavy-duty slide rails for 440, 500 and 5000 frame series. Standard post mount 48 to 215 frame. Pivot/bolted adjustable base 254 to 445 frame. Arrangements 9H and 9SS: clockwise with motor position L. The chapter's worked example uses a 326T motor on Arrangement 9 (NEMA slide base) and a 5008S motor on Arrangement 1 (heavy-duty slide rails), and notes that a 449T motor needs heavy-duty slide rails.

Motor Positioners

A motor base sets belt tension, but the motor also has to be aligned: its shaft square to the drive and, on a coupled or sheave drive, in line with the mating shaft. A motor positioner is a threaded jacking screw mounted next to the motor that pushes the motor a controlled distance so it can be aligned. FanPedia also calls it a motor alignment jacking screw, defines the basic motor positioner as providing horizontal adjustment of the motor position in one direction (FanPedia p. 190), and lists positioners among the pedestal accessories used next to bearings or motors to aid alignment during assembly and testing (FanPedia p. 130). The bearing positioner in Chapter 16 is the same idea applied to fan bearings.

FanPedia defines three more variants, each adding one direction of adjustment (FanPedia pp. 148 and 190):

  • A bi-directional positioner provides horizontal adjustment of the motor position in both directions of the horizontal plane, so the motor can be moved along and across the base.
  • Vertical jack screws thread through the motor feet, which the motor vendor drills and taps. Turning them lifts the motor so shims can be set under its feet to correct height and angle. The vertical jack screws are removed after the motor is shimmed.
  • A tri-directional positioner combines horizontal positioners with vertical jack screws, giving adjustment in both horizontal directions and vertically. Because it uses vertical jack screws, its motor feet are also drilled and tapped by the vendor, and those screws are likewise removed after shimming.

The comparison below restates the four from the list above side by side.

Positioner Directions of adjustment Motor feet drilled and tapped by vendor Removed after shimming
Motor positioner Horizontal, one direction Not noted Not noted
Bi-directional positioner Horizontal, both directions Not noted Not noted
Vertical jack screws Vertical Yes Yes
Tri-directional positioner Horizontal, both directions, plus vertical Yes Yes (the vertical jack screws)

Tapped Motor Feet Start at the Motor Vendor

Breeze warning Tri-directional positioners and vertical jack screws need motor feet that the motor vendor drills and taps, so they have to be on the motor order from the start. Adding them after the motor has shipped means reworking the motor. Put the positioner on the same line as the motor, not on a later accessory list.

Putting the Motor Line Together

Every section of this chapter adds one field to the motor line or one item to the mounting. A single example shows how they combine.

Worked example: a complete motor description. A paper mill needs an Arrangement 9 housed centrifugal fan for a process exhaust. The selection calls for 21 bhp at the fan, and the fan will run on a VFD supplied by the mill. The mill's standard requires IEEE 841 motors, premium efficiency and winding RTDs, on 460 V, three-phase, 60 Hz power. The area is not classified hazardous.

  1. Power: 21 bhp needs a 25 HP motor; the motor runs near 84% load, inside FanPedia's 50% to 100% band. Because the motor is on a VFD, do not count on a service factor above 1.0.
  2. Speed: a 4-pole motor, 1800 RPM synchronous on 60 Hz; the belt drive sets the fan speed.
  3. Technology and efficiency: induction, premium efficiency (IN, P).
  4. Electric power: 3 phase, 60 Hz, 460 V, a single stated voltage.
  5. Duty and enclosure: IEEE 841, which requires TEFC (IP56).
  6. Frame: 284T, the NEMA frame for this horsepower and speed in a T-frame TEFC motor (confirm against the motor data sheet).
  7. Modifications: inverter-duty rating for VFD operation, winding RTDs, and a shaft grounding ring or insulated bearings as agreed with the mill.
  8. Mounting: 284 is within 48 to 445 and Arrangement 9 is on the list, so a NEMA type slide base on the pedestal.

The motor line reads: 25 HP, 1800 RPM, IN, P, 3 Phase, 60 Frequency, 460 Volt, IEEE 841, TEFC, 284T, inverter duty, winding RTDs, shaft grounding ring; NEMA slide base.

Key Takeaways

  • Synchronous speed is \( n_s = 120 f / p \): 1800 RPM for a 4-pole motor on 60 Hz, 1500 RPM on 50 Hz; slip makes actual speed slightly lower.
  • State phase, frequency and one site voltage, even for a dual-voltage motor, whenever the fan ships with electrical add-ons.
  • Premium efficiency compares to NEMA Premium (IEC IE3); efficiency peaks near 75% load and falls off below 50%.
  • Fan motors typically have Class F insulation with a Class B temperature rise. Thermistors and thermostats trip on overload, locked rotor and high ambient; winding RTDs report a continuous temperature.
  • Intended duty: GD for clean, cool air; SD where severe duty is needed and IEEE 841 is not specified; IEEE 841 when specified; DFG explosion proof for the owner's NEC hazardous classification, which does not replace spark-resistant fan construction.
  • A VFD changes frequency and so speed: airflow follows the speed ratio, pressure its square, power its cube. Never exceed the catalog maximum safe speed.
  • Inverter-duty motors handle VFD voltage spikes and shaft currents (NEMA MG-1 Part 31); service factor on a VFD is commonly 1.0.
  • Bases: NEMA slide base 48 to 445 frame, heavy-duty slide rails for 440, 500 and 5000 series, standard post mount 48 to 215, pivot/bolted 254 to 445, automotive pivot bases with rotation-restricted motor positions.
  • Positioners align the motor in one direction, both horizontal directions, vertically (jack screws) or all three; vertical jack screws need vendor-tapped feet and come out after shimming.

You Can Write the Whole Motor Line

Breeze celebrating You can now take a duty point, a plant's power and its environment, and turn them into a complete motor description with its base and positioners. That line is one of the most frequently corrected parts of a fan order, and you know where every field comes from.

See Annotated References